The full potential of electron beam radiation therapy can only be realized when accurate 3-D algorithms for dose computation are available for general clinical use. This proposal is to develop such a system. Making use of the most recent advances in the physics of electron beams, the system will calculate treatment parameters. The calculational methods will be based on a pencil beam model, which uses: a) Fermi-Eyges theory modified for the loss of electrons with depth to calculate the pencil beam spread; b) the central axis dose distribution model which individualizes beam characteristics to a given accelerator; c) the theory of non-equlibrium electron transport at tissue interfaces; and d) multiple slices of body cross-sectional data. Three dimensional dose distributions will be calculated by adding together, as a weighted sum, the dose contributions from an array of field segments in the shape of columns of rectangular cross section, each treated as a simply layered phantom. Appropriate phantoms will be constructed to evaluate the accuracy of the physical models and treatment planning system. Computer software developed in the course of this project will be readily exportable to other institutions for clinical use.